I have solved all the puzzles and my game is complete, yet all the children are still curious about the cactus. Is this just something that they will always do, or can something further be done with the cactus? I seem to recall a post saying that something subtle happens with the cactus, but I havent noticed anything. am I just short-sighted, or is there something I need to do?

Each time you take a villager to the catus and he/she doesn't shake their head "no", they improve at healing.

And if you don't take them, and they haven't gone because they're doctors, then they'll go for the same reason people climb mountains -- because it's there

That change in the cactus occurs only in Village Sim, the Palm version of the game.